1. Field of the Invention
The present invention relates to a magneto-optical recording medium for use in a magneto-optical memory, magnetic recording, or display element to read a record by utilizing a magnetic optical effect such as the magnetic Kerr effect or Faraday effect.
2. Related Background Art
In the magneto-optical recording, the information is written (or recorded) in such a manner as to cause a laser beam to irradiate a surface of a magnetic layer under the application of a bias magnetic field to a magnetic layer region of a magneto-optical recording medium. The laser heats an irradiated portion to a temperature near a Curie point or compensation point of the magnetic layer, and orients the magnetization of this portion in a direction opposite to that of other portions. On the other hand, the reading (regeneration) of written information is performed in such a manner as to cause a laser beam having a lower output than that used for recording and linear polarization, to irradiate a surface of the magnetic layer, and to optically detect the rotation performed by the magnetic optical effect on a polarized plane of reflected light via an analyser.
As the magneto-optical recording medium for use in such a magneto-optical recording, it is desirable to have a high writing efficiency, a stable storage of written information, and excellent reading characteristics. However, it is very difficult to meet all of these requirements with a single material. For example, in writing at the Curie point, it is necessary to decrease the Curie temperature in order to raise the writing efficiency, while the decreased Curie temperature may bring about the degradation of reading characteristics due to the lowered magnetic optical effect. Also, in writing at the compensation point, it is possible to write onto a material of a high Curie temperature with a relatively low power, as the material whose coercive force rapidly decreases near the compensation point is used, but there is a problem associated with the preserving stability of written information and the shape of magnetic domains.
Therefore, conventionally, a magneto-optical recording medium with the composite film having separate functions has been proposed. That is, a magnetic layer having a large magnetic optical effect and an excellent reading characteristic and a magnetic layer having a relatively low Curie temperature, a large vertical magnetic anisotropy, and an excellent writing characteristic are magnetically coupled to make a laminate, whereby the information written into the layer having the excellent writing characteristic is transferred onto the layer having the excellent reading characteristic with a magnetic coupling force to read out this transferred information. Particularly, the composite film which is coupled with an exchange coupling force is favorable in the transferability of the information and the preserving ability of transferred information, wherein a magneto-optical recording medium using Gd-Fe/Tb-Fe exchange coupling double layer film or Tb-FeCo/Tb-FeGo exchange coupling double layer film has been disclosed in Japanese Patent Application Laid-Open No. 57-78652 or No. 63-302448.
However, in the magneto-optical recording medium with the conventional composite film as above described, the magnetic layer having a large magnetic optical effect and an excellent reading characteristic is constituted of a material or composition with which the magnetic domains produced by the writing cannot be stably held with a single layer. That is, this magnetic layer can form and preserve the magnetic domains well only by the exchange coupling with the magnetic layer superior in the writing characteristic. Therefore, as its reaction, an action will be exerted in a direction collapsing the magnetic domains, due to the exchange coupling with the unstable magnetic layer (magnetic layer superior in the reading characteristic) of magnetic domains, in the magnetic domains formed in the magnetic layer superior in the writing characteristic. Accordingly, with the magneto-optical recording medium having conventional composite film, there was a problem that the preserving stability of the information was inferior to a medium where the layer for reading was not laminated. Particularly, it was inferior in the durability on repeated regeneration, or regenerative durability.
Also, in order to obtain a sufficient improvement of the reading characteristic, it is necessary to make the layer for reading thicker to some extent, but there was a problem that if the film thickness of this layer was larger, excellent magnetic domains could not be formed, or the preserving stability of the magnetic domains was worse.
In order to avoid these problems, it can be conceived that the characteristics of the reading layer should be those capable of stably preserving the magnetic domains formed, even when its layer exists singly. For its purpose, it is necessary to constitute this layer of a material or composition having a large vertical magnetic anisotropy and a relatively small saturated magnetization.
However, in this case, it is very difficult to implement a process of transferring the information written into the layer for writing into the layer for reading, and particularly, there is a problem that it is impossible to make the film thickness of the reading layer sufficiently large for reproduction.